1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and, more particularly, to an active matrix type TFT LCD having a TFT (thin film transistor) as a switching element at each pixel.
2. Description of the Related Art
The market of LCDs is rapidly growing because LCDs have become able to provide display quality that sufficiently allows them to replace CRTs which have been typical displays in the related art. They are used as displays of various viewers, cellular phones, PDAs (personal digital assistants), and notebook type personal computers as well as for monitors of desktop computers and television receivers for home use, thanking to their advantage of being flat panels. Thus, LCDs are used as displays that provide from small screens having diagonal dimensions of about 2 inches to large screens having diagonal dimensions in the excess of 40 inches. More and more LCDs are used in various fields as full-color displays capable of displaying still image and dynamic images.
Referring to the trend in LCD techniques, the main stream has shifted from the passive matrix type that has no switching element in a pixel to the active matrix type that has switching elements such as TFTs. Further, referring to the material of channel regions (active semiconductor layers) of TFTs formed in pixels of active matrix type LCDs, a-Si (amorphous silicon) is being replaced by p-Si (polysilicon) having higher carrier mobility.
Structure of TFT LCDs will now be briefly described. For example, in the case of a transmission type TFT LCD that employs a back-light unit, a TFT substrate (array substrate) that is a transparent insulated substrate such as a glass substrate and an opposite substrate are combined in a face-to-face relationship with a predetermined cell gap therebetween, and a liquid crystal is sealed between the substrates. A plurality of pixel electrodes are provided in the form of a matrix on the TFT substrate, and the TFT is connected to each of the pixel electrodes. A common electrode is formed on the opposite substrate. In the case of a color display LCD, a color filter (CF) is formed on either the TFT substrate or the opposite substrate. Alignment films are formed at interfaces between the substrates and the liquid crystal layer. Polarizers having a crossed Nicols configuration, for example, are applied to the outside of both substrates.
FIG. 7 is an equivalent circuit for one pixel of a TFT LCD in the related art. As shown in FIG. 7, a gate electrode G of a TFT is connected to a gate bus line Lg. A source electrode S of the TFT is connected to a pixel electrode Pe, and a drain electrode D is connected to a data bus line Ld. A liquid crystal layer lc is sandwiched by the pixel electrode Pe and a common electrode Ce to form a liquid crystal capacitance Clc. A storage capacitor Cs is connected in parallel with the liquid crystal capacitance in practice, although not shown.
A gate voltage Vg is applied to the gate bus line Lg from a gate bus line driving circuit that is not shown. A grayscale voltage Vd is applied to the data bus line Ld from a data bus line driving circuit that is not shown. A common voltage Vcom (=0 V) is applied to the common electrode Ce.
The liquid crystal lc is positively or negatively anisotropic in its dielectric constant, which results in a property that the liquid crystal molecules rotate in accordance with the strength of an electric field applied thereto. The liquid crystal lc is also anisotropic in its refractive index, which results in a property that the polarization of light passing through the liquid crystal lc changes in accordance with the rotation of the liquid crystal molecules. Therefore, when a voltage is applied between the pixel electrode Pe and the common electrode Ce, the liquid crystal molecules rotate in accordance with the value of the applied voltage, which results in a change in the polarization of the light that has been linearly polarized by the entrance side polarizer in the liquid crystal lc. The quantity of the light that passes through the polarizer at the light emitting side is thereby adjusted to display a tone.
While a voltage of about 5 V can be applied to common liquid crystal materials, when an electric field is continuously applied to the liquid crystal lc only in one direction, the liquid crystal material will be degraded. In order to prevent this, the electric field for driving the liquid crystal is applied to the liquid crystal lc with the polarity thereof inverted in a predetermined cycle. In general, a frame inversion driving is used in which the polarity is inverted in the cycle of display frames.
A separate pixel electrode Pe is provided for each pixel, and a single electrode is provided as the common electrode Ce such that it will be shared by all pixels. A driving method as shown in FIG. 8 is used to achieve the frame inversion driving with utilizing such common electrode Ce. FIG. 8 shows time in the horizontal direction and voltages in the vertical direction to indicate a relationship between the gate voltage Vg, the grayscale voltage Vd, and the common voltage Vcom.
As shown in FIG. 8, the common voltage (the potential at the common electrode) Vcom (=0 V) is constant. The grayscale voltage Vd that ranges up to ±2.5 V of the common voltage Vcom is applied to the data bus line Ld. FIG. 8 shows a state in which the grayscale voltage Vd (data) having an absolute value V0=2.5 V is output on the data bus line Ld in each frame fn, the polarity of the voltage being inverted in each frame fn.
When an n-channel type TFT connected to the gate bus line Lg is to be kept off state, a potential Vg (off) is output which has an absolute value that is smaller than the maximum negative polarity grayscale voltage Vd=−V0 (V) by V1 (absolute value). When the TFT is to be kept on state, a potential Vg (on) is output which has an absolute value that is greater than the maximum positive polarity grayscale voltage Vd=+V0 (V) by V2 (absolute value). That is, a gate pulse having a potential Vg=Vg(on) is output to the gate bus line Lg during a period in which the TFT is made to be on state. The height of the gate pulse is V1+2×V0+V2. The voltage V1 must be increased when the off-current is poorly disconnected, and the voltage V2 must be increased when the on-current is small for reasons associated with the property of retaining accumulated charges and the data rewriting speed. Therefore, a driving voltage of about 13 V is normally used such the TFT will be reliably turned on and off regardless of the polarity of the same.
As thus described, a power supply circuit of 13 V is required to drive the TFT LCD in the related art in spite of the fact that the maximum grayscale voltage Vd required for writing the pixel electrodes Pe is 2.5 V. The driving voltage of 13 V is applied not only to the gate bus line driving circuit but also to switching elements in the data bus line driving circuit for controlling the flow of signals output to the data bus lines Ld. The maximum driving voltage depends on the liquid crystal material used, and some TFT LCDs require a driving voltage of 16 V or 18 V or more that is higher than the voltage in the present example.
As thus described, in the TFT LCD in the related art, the power supply voltage for the gate bus line driving circuit and the data bus line driving circuit for driving the liquid crystal lc at each pixel is very much higher than the voltage band of 5 V applied to the liquid crystal lc. Therefore, the TFT must have a high gate withstand voltage and drain withstand voltage. This results in a need for countermeasures including an increase in the thickness of gate oxide films of the TFT, an increase in the channel length, and an increase in the LDD (lightly doped drain) length. However, such countermeasures result in an increase in fluctuation of a threshold voltage Vth of the TFT and a reduction in the on-current of the TFT. Consequently, a further increase of the driving voltage will be required to achieve proper operations in the presence of a great fluctuation of the threshold Vth, and still further increase of the driving voltage will be required to achieve a required switching speed while suppressing any reduction in the on-current. This only results in a vicious cycle, and no reduction of the driving voltage can be achieved. An increase of the driving voltage is problematic also in that it leads to an increase of power consumption and an increase of electromagnetic interference with environment.
The recent establishment of low temperature polysilicon manufacturing processes has made it possible to fabricate a FET having a channel region formed from p-Si (polysilicon) on a member having a relatively low melting point such as a glass substrate. It therefore becomes possible to fabricate a TFT substrate integral with peripheral circuits in which various circuits including a gate bus line driving circuit and a data bus line driving circuit are incorporated in peripheral regions of a glass substrate on which pixel TFTs are to be fabricated. FETs of the peripheral circuit sections must be formed with a gate length that is as small as possible to allow an operation at a high speed, and they must inevitably be of a low voltage drive type. Further, balanced circuits that consume low power cannot be obtained unless they are of a low voltage drive type.
When the pixel TFTs are of a high voltage drive type, a mixture of the low voltage drive type FET's and high voltage drive type TFTs must be formed on a single glass substrate, which results in a problem in that the manufacturing process will become complicated and the manufacturing cost will increase. Therefore, to manufacture a TFT substrate integral with peripheral circuits utilizing a low temperature polysilicon manufacturing process, the driving voltage of the pixel TFTs must be reduced to become as close as possible to the driving voltage of the FET's of the peripheral circuits.